In one known type of injection molding apparatus for manufacturing shoes having shoe soles made, for example, of polyurethane, there is employed an upper mold consisting of a last for receiving a shoe shaft, a sole-heel mold longitudinally divided into side mold and a bottom stamp to form a hollow mold chamber. The raw materials for forming the shoe sole is fed into the hollow mold chamber by an injection apparatus which can be brought into association with the molds at various stations arranged around a rotatable table. An example of such an injection molding apparatus is disclosed in U.S. Pat. No. 4,286,936 to H. Hustedt.
In the U.S. Pat. No. 3,676,542 to F. V. Maltby, there is disclosed a method of manufacturing shoes having woven textile uppers wherein a last is formed with recesses extending along its bottom portion adjacent the peripheral part of the sole portion of the last. Electric heating elements are embedded in the recesses for heating the margin of the sole of the last so as to heat the adjacent canvas of the upper before contact by the molten thermoplastic material that is injected into the mold cavity. The peripheral sole portion of the last is heated in such manner to above the temperature which the textile upper would reach by the natural transfer of heat by an injection molding operation. During such heating of the peripheral part of the sole portion of the last, the patent describes that the middle part remains at a lower temperature. The patent states that the heating step provides good penetration of the canvas of the textile upper by the thermoplastic material and satisfactory adhesion between the upper and the sole.
Also, in the U.S. Pat. No. 3,441,643 to Tusa et al, there is described a method of preheating the sole portion of a textile upper by a heating element embedded in the last to condition the textile component to penetration by hot thermoplastic material for adhesion between the plastic and the textile. In both the Maltby and Tusa et al injection molding systems, the heating elements are embedded in the last and thereby retain a significant amount of heat in the last even after the adhesion process between the sole and the textile upper. This is disadvantageous in that there is some loss of control over the heat supplied to the last and sole even after the preheating operation until the formed shoe is physically removed from the heated last. This also affects the rate of cooling of the completed shoe and may result in undesirable shoe formation.
U.S. Pat. No. 3,085,294 to F. Rosenbaum describes a method of securing a leather, fabric or plastic lasting to a sole made of a vulcanizable rubber compound where the rubber sole is pre-heated before the molding and vulcanization stage by radiant heating elements located on opposite sides of the rubber sole surface for causing the sole to soften to a moldable plastic condition. The patentee states that the purpose of such pre-heating of the rubber sole is to enable the lower rim of the lasting margin to be quickly and fully embedded in the rubber sole as required before vulcanization can take place to any large extent.
U.S. Pat. No. 3,555,609 to Chu et al describes an injection molding apparatus for making shoes wherein the sole mold is provided with a coolant fluid during the injection process, prior to the removal of the lasting in its completed shoe form from the last. While such cooling is said by the patentee to reduce production time, cooling in this manner during the injection process before the shoe is completely formed may result in undesirable shoe formation.
It is an object of the present invention to provide a method and apparatus for manufacturing shoes using an injection molding apparatus wherein the lasting is of the type comprising a thermoplastic layer and is securely bonded to the sole and heel. It is another object to provide a method and apparatus for manufacturing shoes in an efficient manner while maintaining control over the desired formation of the shoe. It is another object to manufacture shoes using injection molding apparatus wherein control of the shoe temperatures both prior to and after the injection molding operation forming the completed shoe provides secure bonding of the lasting with the sole as well as maintaining the desired shoe shape after the injection molding operation.